AC not cooling: the diagnostic order that saves a service call
Contents (8)
« My AC is not cooling » covers three different failures, and naming yours first saves time and money: no air at all at the registers, plenty of air that is not cold, or cold air that never wins against the house. The first two usually have a cause a homeowner can find in twenty minutes; the third is frequently not a fault at all. A meaningful share of summer service calls end with a technician replacing a filter or resetting a safety switch — and billing the visit — so the order below catches those first.
Start by naming the symptom
- Nothing runs at all. No indoor blower, no outdoor fan. Look at power, thermostat and safety switches.
- Indoor blower runs, outdoor unit does not. Classic capacitor, contactor or condensate-switch territory.
- Both run, the air is weak or lukewarm. Airflow restriction, a frozen coil, a smothered condenser, or a low charge.
- Both run, the air is genuinely cold, the house is not. The load exceeds the capacity — sizing, ducts, envelope, or simply a design-day afternoon.
The checks you can safely do yourself
1. The thermostat — two minutes
Set the mode to COOL rather than an auto-changeover setting, put the setpoint at least 5 °F below the current room reading, and set the fan to AUTO rather than ON. Fan ON pushes room-temperature air through the registers between compressor cycles and is the single most common « it blows warm air » false alarm. Replace the batteries in a battery-powered stat: a dying one stops calling for cooling well before its display goes blank. And check for a schedule or an eco hold that someone programmed and forgot.
2. The air filter — five minutes
A clogged filter starves the evaporator of airflow, which drives the coil below freezing, which builds ice, which restricts airflow further. It is the classic cause of « runs constantly, barely cools ». Pull the filter and hold it up to a light: if no light comes through, replace it. In dusty or pet-heavy homes that means every 30 to 60 days during cooling season, not once a year.
3. Power and breakers — five minutes
A central system has two circuits — air handler and outdoor condenser — plus a service switch near the furnace that looks exactly like a light switch and gets flipped by accident, plus a pull-out disconnect beside the outdoor unit. Reset a tripped breaker once. If it trips again immediately, stop: a repeat trip is a fault report, not an inconvenience.
4. The condensate drain and float switch — five minutes
Most modern installs carry a safety switch that shuts the system down when the drain pan fills, precisely so that you get a warm house instead of a ruined ceiling. A biofilm-clogged drain line is the usual culprit: clearing it with a wet/dry vacuum at the outdoor termination, then flushing a cup of distilled vinegar through the access tee, restores service in many cases. The tell-tale pattern is a system that cools for fifteen or twenty minutes, shuts off, and leaves standing water in the pan.
5. The outdoor unit — ten minutes
Cut power at the disconnect first. Then look at the coil: cottonwood fluff, grass clippings and dryer lint mat into the fins and choke heat rejection, and a condenser that cannot reject heat cannot make cold air indoors. Two feet of clearance on every side is the working rule. Rinse from the inside outwards with a garden hose, never a pressure washer, which folds the fins flat. A fan that hums without turning, visibly bent fins, or a burnt smell are parts problems, not housekeeping.
6. Registers and returns
Closed supply registers do not save money — they raise duct static pressure and cut total airflow; blocked returns do the same in reverse. One hot room in an otherwise comfortable house is a distribution problem, not a machine problem.
If the coil is frozen
Ice on the refrigerant line at the indoor unit, or a solid white block where the evaporator sits, means one of two things: not enough air moving across the coil, or not enough refrigerant inside it. The immediate procedure is identical either way. Switch cooling OFF at the thermostat and set the fan to ON. Allow four to twenty-four hours for the ice to clear with the blower moving air over it — running a compressor against a block of ice is how a repair becomes a replacement. Then fit a clean filter, open every register, and restart. If it freezes again within a day on a clean filter, the cause is the charge, and the service call is no longer optional.
Symptom to cause, at a glance
| Symptom | Most likely cause | Who fixes it | Indicative cost |
|---|---|---|---|
| Warm air between cooling cycles | Thermostat fan set to ON | You | $0 |
| Weak airflow, endless run times | Clogged filter or iced coil | You | $10–$30 |
| Runs, stops, water in the pan | Blocked drain, float switch tripped | You, or a tech | $0–$250 |
| Indoor fan runs, outdoor does not | Run capacitor or contactor | Technician | $150–$450 |
| Cools, then ices up, repeatedly | Refrigerant leak | Certified technician | $400–$1,500+ |
| Loud hum, breaker trips, no start | Compressor or hard electrical fault | Technician | Repair-or-replace decision |
Those bands are market reference ranges for a typical US residential job, not quotes: regional labour rates move them substantially, and any figure that arrives before a diagnosis is a sales number rather than an estimate.
Where the DIY zone ends
Three things belong to a technician without exception. Capacitors hold a dangerous charge long after the power is cut. The sealed refrigerant circuit may only be opened, recovered and charged by technicians certified under EPA Section 608 rules. And a breaker that trips twice is an electrical fault, not a reset. Everything upstream of that line — filters, drains, coil rinsing, thermostat setup, clearance around the condenser — is homeowner work, and it covers most of what actually goes wrong.
The refrigerant myth
An air conditioner does not consume refrigerant. A sealed system that is low is a system that leaks, so « we topped it off » with no leak search is a subscription rather than a repair. Ask for the leak to be located and the new charge weighed in rather than « added until it looked about right ». This is expensive advice to ignore in 2026: R-410A production for new residential equipment wound down under the AIM Act from 2025, new systems ship with lower-GWP refrigerants such as R-454B and R-32, and recharging an older system costs more every season — the trajectory that tilts our replacement cost breakdown.
When it is not a fault at all
A properly sized system is designed to hold roughly 20 °F below the outdoor temperature at the design condition, and to run almost continuously on the hottest afternoons. Continuous running during a heatwave is the machine working, not failing. Two measurements separate « not enough machine » from « broken machine ». The first is the temperature split between return and supply air, which sits around 16–22 °F on a healthy system: measure it with any kitchen thermometer at a return grille and at the nearest supply register. The second is indoor humidity, which should fall below about 55 % while the system runs. A healthy split in a house that stays hot points at capacity, ducts or envelope — check the sizing arithmetic against our BTU sizing guide and the cooling degree days on your state page. A split of 5 °F points squarely at the equipment.
Ductwork is the most under-diagnosed cause of « the AC is fine but the house is hot »: supply runs crossing a 130 °F attic give back much of the cooling they carry, and leaky returns up there import hot air into the system. Sealing them usually buys more comfort per dollar than any equipment change, so it belongs in the conversation before a new central system is quoted.
Frequently asked questions
Why does my AC blow warm air only some of the time?
Two classic patterns. With the fan set to ON, air keeps moving between compressor cycles and that air is simply room temperature — switch to AUTO. Alternatively the system cools normally, ices over an hour or two, then blows warm until it thaws; that repeating cycle is the signature of restricted airflow or a low charge.
Is it safe to keep running the AC with a frozen coil?
No. Liquid refrigerant returning to a compressor built to receive vapour is the fastest route from a modest repair bill to a full system replacement. Turn cooling off, leave the fan running, and wait for the ice to clear completely before restarting.
How long should it take to cool the house down?
From a hot start on a 95 °F afternoon, expect several hours rather than minutes, and expect the final few degrees to be the slowest. If the setpoint was raised 10 °F while the house was empty, a pull-down of that size on a design day may simply not finish before evening — a scheduling issue rather than a fault.
Does an annual tune-up actually prevent these failures?
It reliably prevents the airflow and drainage half of them: coil cleaning, drain flushing, capacitor testing and charge verification catch most of the list above before July. It cannot prevent a compressor reaching the end of its service life. Treat spring maintenance as insurance against the avoidable failures, not against all of them.
The outdoor unit hums but the fan blade is not turning. What is that?
Almost always a failed run capacitor, occasionally a seized fan motor. It is a common and inexpensive part — and also the component most likely to be sitting at a dangerous voltage after you cut the power. Leave it to a technician; the swap is typically a same-day, low-cost repair.
Sources
Read next
- Ice on the coil: why an air conditioner freezes up
- HVAC tax credits and rebates: claiming what you are owed
- Ceiling fans and air conditioning: what the setpoint offset really buys
- AC keeps tripping the breaker: reading the timing before you reset
- What it costs to run an air conditioner in the United States